1. Field of the Invention
The present invention relates to a thiophene-containing compound and a polymer thereof useful as materials for organic electronic devices such as electrophotographic photoreceptors, organic electroluminescent devices and organic transistors. Specifically, the invention relates to a thiophene-containing compound and a polymer thereof superior in charge-transporting characteristics and light-emitting characteristics.
2. Description of the Related Art
As charge-transporting materials, charge-transporting polymers, a typical example of which is polyvinylcarbazole (PVK), and low molecular weight compound dispersed systems, in which a charge-transporting low molecular weight compound is dispersed in a polymer, are well known. Low molecular weight dispersed systems are mainly used, particularly for electrophotographic photoreceptors since the systems are variable in raw material thereof and high function can be obtained easily.
In recent years, electrophotographic photoreceptors have been used in high-speed copiers or printers as the performance of organic photoreceptors has become more advanced. However, the performance is not necessarily sufficient at present, and a longer life span thereof is earnestly required. As for the current mainstream charge-transporting layer of low molecular weight systems, the electric properties thereof are becoming sufficiently satisfactory. However, the charge-transporting layer has a problem in that the layer is essentially poor in mechanical strength and is weak against abrasion since a low molecular weight compound is dispersed in a polymer.
In organic electroluminescent devices, low molecular weight charge-transporting materials are generally used by vapor-deposition. However, since organic electroluminescent devices are driven with high current density, i.e. some mA/cm2, a huge amount of Joule heat is generated, whereby morphologic changes can easily occur, leading to phenomenon such as decrease in brilliance and dielectric breakdown. As a result, the life span of devices is shortened. As for high molecular weight charge-transporting materials, there are not so many materials having both charge-transporting properties and light emitting properties, and therefore, using such materials has been problematic in view of efficiency and life span of devices.
On the other hand, charge-transporting polymers, a typical example of which is PVK, are actively researched at present as a photoconducting material for electrophotographic photoreceptors, and also actively researched as a charge-transporting material since there is a possibility that the above-mentioned drawbacks of the polymers can be greatly overcome as described in, for example, Proceedings of the 37th Applied Physics Joint Meeting 31p-K-12, 1990, the disclosure of which is incorporated by reference herein. For example, U.S. Pat. No. 4,806,443 discloses a polycarbonate obtained by polymerizing a specific dihydroxyarylamine and bischloroformate, and U.S. Pat. No. 4,806,444 discloses a polycarbonate obtained by polymerizing a specific dihydroxyarylamine and phosgene (the disclosures of U.S. Pat. Nos. 4,806,443 and 4,806,444 are incorporated by reference herein).
Moreover, U.S. Pat. No. 4,801,517, the disclosure of which is incorporated by reference herein, discloses a polycarbonate obtained by polymerizing bishydroxyalkylarylamine, and bischloroformate or phosgene, and U.S. Pat. Nos. 4,937,165 and 4,959,228, the disclosures of which are incorporated by reference herein, disclose a polycarbonate obtained by polymerizing a specific dihydroxyarylamine or bishydroxyalkylarylamine, bishydroxyalkylamine, and bischloroformate, or a polyester obtained by polymerizing the amine and bisacylhalide. Furthermore, U.S. Pat. No. 5,034,296, the disclosure of which is incorporated by reference herein, discloses a polyester, or a polycarbonate of an arylamine having a specific fluorene skeleton, and U.S. Pat. No. 4,983,482, the disclosure of which is incorporated by reference herein, discloses a polyurethane. Additionally, Japanese Patent Application Publication (JP-B) No. 59-28903, the disclosure of which is incorporated by reference herein, discloses a polyester having, as a main chain, a specific bisstyrylbisarylamine.
Japanese Patent Application Laid-Open (JP-A) Nos. 61-20953, 1-134456, 1-134457, 1-134462, 4-133065 and 4-133066, the disclosures of which are incorporated by reference herein, suggest polymers and photoreceptors having, as a pendant, a charge-transporting substituent such as hydrazone and triarylamine.
Organic electroluminescent devices using π-conjugated polymers represented by paraphenylenevinylene (PPV) (see, for example, Nature, Vol. 357, 477, 1992, the disclosure of which is incorporated by reference herein), and organic electroluminescent devices using polymers having triphenylamine introduced into the side chain of polyphosphazene are proposed (see, for example, Proceedings of the 42nd Polymer Forum 20J21, 1993, the disclosure of which is incorporated by reference herein).
Various properties such as solubility, film-formability, mobility, heat resistance, and matching of oxidation potential are required of charge-transporting materials. In order to satisfy these requirements, generally a substituent is introduced to the polymer so as to control the physical properties thereof.
Since the physical properties of the charge-transporting polymer have high correlation with the physical properties of a charge-transporting monomer, which is a raw material of the polymer, the molecular design of the charge-transporting monomer (low molecular weight material) becomes important.
The monomer which is the raw material of the above-mentioned triarylamine polymer can be roughly classified into two kinds, that is, (1) dihydroxyarylamine, and (2) bishydroxyalkylarylamine.
However, since dihydroxyarylamine of (1) has an aminophenolic structure, it is easily oxidized and is not easily purified. Particularly in a case in which dihydroxyarylamine has the hydroxy group at para position, the dihydroxyarylamine becomes more unstable. Additionally, dihydroxyarylamine has a structure in which oxygen of the substituent bonds directly to the aromatic ring; therefore, charge distribution is easily unbalanced by the electron-withdrawing property of the oxygen. As a result, a problem occurs in that the mobility thereof falls easily.
On the other hand, bishydroxyalkylarylamine of (2) is not affected strongly by the electron-withdrawing property of oxygen because of the methylene group, but monomer is not easily synthesized. That is, according to a reaction of diarylamine or diarylbenzidine with 3-bromoiodobenzene, a mixture tends to be produced since both bromine and iodine have reactivity. Thus, the yield of the target amine falls. The following problem also arises: alkyllithium, which is used when lithium is bonded to a bromine, or ethylene oxide is highly dangerous and toxic and therefore requires careful handling.
The π-conjugated polymers represented by PPV described above, and the organic electroluminescent devices having triphenylamine introduced into the polyphosphazene side chain involve problems in color tone, luminous intensity and durability.
Thus, there is a need for development of an organic electronic material which can be synthesized easily, exhibits excellent charge-transporting performance and is superior in light-emitting characteristics, in order to develop organic electronic devices, such as organic electroluminescent devices, that have greater luminescence brightness and are superior in stability during repeated use.